Hygrometric elements



Feb. 18, 1964 A. c. JAsoN ETAL 3,121,853

HYGROMETRIC ELEMENTS Filed March 29. 1961 M-Mu.

- INV'E1:TORS: ALFRED CHARLES JSDN .ALEXANDER LEES than 200 or a diameter in excess of -7 oms.

United States Patent 3,121,853 HYGRGMETRIC ELEMENTS Alfred Charles Jason and Alexander Lees, both Torry Research Station, Aberdeen, Scotland Filed Mar. 29, 1961, Ser. No. 99,308 14 Claims. (Cl. 3258-35) This invention relates to hygrornetric elements ott which the humidity responsive element is a layer of porous metallic oxide, the electrical properties of which layer (notably the effective capacitance and resistance) vary with variations in humidity, the layer being included between electrodes one of which is porous. Thus it has been proposed to provide a base of conducting material (eg. a plate of aluminium) with a thin ilm of metallic oxide (eg. :aluminium oxide produced ele-ctrolytically) and a thin metallic foil such as gold leaf or a layer of gold, silver or other metal deposited by evaporation or cathode sputtering, the foil being thin enough to permit absorption and dissipation of moisture through it by the oxide layer. British patent specification No. 732,969 discloses the use in a similar element, of a coherent conductive vapour-permeable deposit oi graphite instead of a thin film of metal. That specification also discloses an element of rod form, the base electrode being an aluminium wire and it further discloses details of electrolytic processes for forming a layer of alumina on an aluminium base which yield elements of high electrical breakdown strength and a low proportion of failures.v

Elements of rod form' produced as described in the said specification No. 732,969 have been found extremely sensitive but have 4also been found to change their electrical pro erties markedly with age when subjected to cycles of high 'and low humidity e..g. 90 to 30 percent, the change always being in the Idirection of lower sensitivity.

An object of the present invention is to mitigate if not arrest this ageing phenomenon. The alumina layer produced as described in the former specification contains numerous line pores extending nearly down to the base and the change in electrical properties with humid-ity is due to a variation in the amount of water adsorbed on the pore sides. This adsorption is believed to be governed by the presence on the pore sides of ions derived from the acid in which the electrolytic oxidation has been performed. Speciication No. 7 32,969 describes the use of chroniic or oxalic acid and British patent specification No. $015,051 the use of sulphuric acid. The loss of sensitivity is believed to be due to the migration of relatively light anions resulting yfrom the electrolytic process, away from the pore sides and into the mass of alumina.

The present invention proposes to reinforce or replace the light anions by a different, heavier ion of much smaller mobility that is `anions having a molecular weight of more The elements produced by the methods described in the abovementioned specification 805,651 or similar methods using sulfplnnic acid are stored for 3 to 6 months at normal humidity (about 60 percent) and temperature. rFhey are then dipped, for a period not less than one second in a saturate-d solution or a salt containing large or heavy anions. Such aV salt is an alkali metal tunlgstate, specifically sodiurn tungstate. The period should ynot exceed one minute, otherwise tungstate crystalsV subsequently form which lead to electrical breakdown of the element.

In addition to the above described treatment, improvements have been made to the mechanical construction or the element.

it will be convenient to describe these with reference to the accompanying drawings illustra-ting two examples both in longitudinal section.

In both cases there is la central stem 11 of aluminium on which an aluminium oxide layer indicated generally at 12 is produced and treated as above described. Over a portion of the stern beyond the end of the oxide layer 12 a thin sleeve 13 of insulating material, preferably an epoxy resin is applied to the unoxidised metal so that it overlaps the oxide .12 for a distance which in the case of a stern of wire of perhaps V16 inch diameter overlaps the oxide for a distance for about l mm.

In the case of FIGURE 1 on to thee insulating sleeve 13 and close tothe portion which overlaps the oxide layer 12 is applied a short thin sleeve 14 of metal such as aluminium. Ths sleeve 14 is secured at both ends by two thin bands 1S, 16 of insulating material applied in the form of a solution, care being taken to make the surface as continuous as possible at the oxide end. The porous film otf conductive metal such as aluminium 17 is then evaporated on to the oxide layer 12 and over the insulating sleeve 13 at this end and is continued sufciently far to make electric contact with the conductive metal sleeve ld. A thin connecting wire 18 is Wound round the exposed part otE the sleeve `1li- -and is protected and secured with a coating 19 of cement, preferably an epoxy resin.

The construction shown in FIGURE 2 is preferred however as it is simpler. ln this case the outer Iiilrn of porous metal indicated at Z0 is first evaporated on to the oxide layer 12 and over the sleeve 13 up to a point about 2 mm. short of the end remote from the oxide, thus leaving the insulation clear for a band of this width. On to this band and on to and in connection with the film coating 2li is applied a thin short sleeve 21 or" metal such as aluminium. A thin connecting wire 22 is wound round this metal sleeve to make connection with it and is protected and secured with a coating 25 of cement, preferably of an epoxy resin.

ln both cases the metal sleeve 14 and Z1 respectively may be produced by evaporation and might be of gold instead of aluminium.

The completed element is treated by the method above described to reinforce or replace the light ions by a different heavier ion of much smaller mobility.

We claim:

1. A method of improving the ageing properties of hygrometric element cornpvrsing a base electrode orf aluminium, a layer thereon of aluminium oxide produced by electrolytic oxidation using sulphuric acid, and an outer porous electrode, in which the element is stored for three to six months at normal room humidity and ternperature, and then dipped for a period not less than a secon-d and not more than a minute in a saturated solution of a. salt containing large or heavy anions.

2. A method according to claim l in which the salt is an alkali metal tungstate.

3. A method according to claim 1 in which the salt is sodium tungstate.

4. A method according to claim 1 in which the element is thereaftercycled between high (eg. 9G percent) and low (eg. 30 percent) humidity until it has aged and its physical characteristics have been adequately stabilised.

5. A hygrometric elem-ent comprising a base-electrode of aluminium, a layethereon of aluminium oxide prodnd by electrolytic oxidation using sulphur-ic acid, and an outer porous electrode, the light ions norrnally present in the sides of the pores in the aluminium oxide being replaced by a diiferent ion of much smaller mobility.

which to make connection with the other electrode, a v

thin insulating sleeve is applied t0 the unoxidised metal of the core electrode so that it slightly overlaps the oxide layer and the outer porous electrode is applied to overlap the insulating sleeve and make contact with a thin metal conducting sleeve upon which a connecting Wire is Wound, the Wire then being protected and secured by insulating cement.

9. A hygrometric element according to claim 8 in which the outer porous electrode overlaps -the metal conducting sleeve but leaves uncovered the portion on which the Wire is Wound. Y

10. A hygrometric element according to claim 8 in which lthe 'conducting sleeve overlaps the outer porous electrode suiiciently to make a good connection to it.

11. A hygrometric element according to claim 10 in `which the insulating sleeve and the protecting and securing coating are both of epoxy resin.

12. A hygronietric element laccording t0 claim 8 in which the conducting sleeve is of evaporated aluminium.

13. A hygrometric element comprising a base electrode of aluminium, a. layer thereon of aluminium oxide produced by eleotrolytic oxidation using sulphuric acid, and an outer porous electrodeproduced by electrolytic oxidation using sulphuric acid, said element being stored for 3 .to 6 months at normal room humidity and temperature, then dipped in a saturated solution of an alkali metal tungstate 'for a period lbetween one second and one minute, and 'then cycled between high and low humidity until its physical characteristics are adequately stabilised.

14. A hygrometric element accord-ing to claim 13 in which the base electrode is an aluminium wire.

References Cited in the tile of this patent UNITED STATES PATENTS Ohlheiser Nov. 4, 19 52 

1. A METHOD OF IMPROVING THE AGEING PROPERTIES OF HYGROMETRIC ELEMENT COMPRISING A BASE ELECTRODE OF ALUMINIUM, A LAYER THEREON OF ALUMINIUM OXIDE PRODUCED BY ELECTROLYTIC OXIDATION USING SULPHURIC ACID, AND AN OUTER POROUS ELECTRODE, IN WHICH THE ELEMENT IS STORED FOR THREE TO SIX MONTHS AT NORMAL ROOM HUMIDITY AND TEMPERATURE, AND THEN DIPPED FOR A PERIOD NOT LESS THAN A SECOND AND NOT MORE THAN A MINUTE IN A SATURATED SOLUTION OF A SALT CONTAINING LARGE OR HEAVY ANIONS. 